1. Field of the Invention
This invention relates generally to optical communication networks, and, more particularly, to circuitry and concomitant methodologies for mitigating transient effects in the networks caused by bursty optical signals.
2. Description of the Background
Commercial-type optical networks, and even exploratory optical networks, have conventionally been designed for optical communication presuming more traditional optical streams, that is, continuous optical streams that are amplitude modulated. Accordingly, optical power is always present in the optical streams.
There is presently substantial interest in extending the utility of the aforementioned networks beyond the traditional approach to optical burst packet transmission and even to optical packet switching. However, optical networks are often intolerant of bursty optical signals and may exhibit rapid time-dependent gain variations which impair transmission of the bursty optical signals. This intolerance arises primarily because of the operating characteristics of certain components deployed in the traditional systems (e.g., Erbium-Doped Fiber Amplifiers (EDFAs)), so that bursty input power resulting from a bursty optical signal (that is, the optical signal is a pulse-type signal with ON and OFF intervals) on one of the channels in the optical network leads to degradation of the bursty data on the given channel and of data (bursty or continuous) on other channels conveyed by the optical network.
As alluded to, EDFA""s are utilized in optical networks to provide gain for propagating optical signals. An EDFA is usually stabilized to improve performance, with two techniques typically used, namely: all-optical gain clamping with a response time of tens of microseconds, and pump-power regulation. There are necessary compromises between the available gain, gain stability, noise figure, dynamic range, and response time in arriving at a suitable EDFA design for network deployment. Rapid changes in optical power can disturb EDFA gain resulting in unwanted modulation of the bursty optical signal as well as other signals simultaneously traversing a given EDFA.
Qualitatively, when an optical burst is transmitted through an optical network optimized for continuous optical streams, the output response signal is characterized by a gain peak followed by gain oscillations for a transient period; this period may be substantial relative to the overall length of the burst. Even with a fast-tracking receiver to receive this response signal, numerous bit-errors arise during the amplitude transient both in the channel carrying the burst as well as other channels. By way of reiteration, this instability occurs because power changes within a single wavelength channel affect the gain for both that wavelength and for other wavelengths through cross-gain saturation in common EDFAs within and between nodes in the optical system.
Cross-saturation effects are especially true if the EDFA is not gain-clamped xe2x80x94such EDFAs are typically used in present-day optical networks. The overall transient response of the optical network due to the bursty input optical signal depends upon the magnitude of the bursty signal, the speed of certain components (e.g., servo-controlled attenuators for stabilizing the EDFAs), the design of the EDFAs, the network topology, and the add/drop characteristics of the network elements, as well as the interactions of the foregoing mechanisms and components.
A recent reference addressing the foregoing issues is a paper entitled xe2x80x9cBurst Optical Packet Transport Over the MONET DC Networkxe2x80x9d, by J. Jackel et al., appearing in the European Conference on Optical Components, September 2000, Post-deadline Paper 2.9. The paper presents a technique for xe2x80x9coptical burst supportxe2x80x9d wherein a second independent but closely space optical ballast signal complements the burst signal""s average power and allows the bursty signal to pass through the network with minimal error. To accomplish this, a second laser is used to generate optical power complementary to the data so that the total input power remains nearly constant. It is a given that the input signal is in electronic form, and the input signal is monitored to determine when to enable the second laser. However, because of the deployment of the second laser, complicated electronics are required, for example, to ensure the delivery of the nearly constant total input power. The additional complicated electronics, as well as the second laser, add to the expense of the overall system. However, in many applications, electronic versions of the input signal are not accessible or even allowed, so in these applications the electronic solution will not work.
Thus, the prior art is devoid of teachings or suggestions relating to all-optical technique for mitigating transients caused by bursty input signals.
Shortcomings and limitations of the prior art are obviated, in accordance with the present invention, by a circuitry and concomitant methodology wherein when a bursty input signal is present (xe2x80x9cONxe2x80x9d interval), conventional amplification of the bursty input signal is effected; when the bursty signal is not present (xe2x80x9cOFFxe2x80x9d interval), oscillation is supported via a semiconductor optical amplifier and an optical feedback loop to generate an optical signal to substitute for the missing signal, thereby presenting nearly constant input power on the optical paths in the optical network.
Broadly, in accordance with a one circuit aspect of the present invention, circuitry for generating a burst support signal to augment an input optical burst signal includes: (a) a semiconductor optical amplifier responsive to the input optical burst; and (b) a wavelength-selective optical feedback circuit having an input coupled to the output of the amplifier and an output coupled to the input of the amplifier, wherein the circuitry produces the burst support signal at the output of the amplifier whenever the power level of the input optical burst signal is below a pre-selected threshold.
The broad method aspect of the present invention is commensurate with this broad circuitry aspect.
Broadly, in accordance with another circuit aspect of the present invention, circuitry for generating a burst support signal to augment an input-optical burst signal includes: (a) an optical combiner having the input optical burst signal as one input; (b a semiconductor optical amplifier coupled to the output of the optical combiner; (c) an optical splitter having an input coupled to the output of the optical amplifier, and an output to emit the burst support signal; (d) a tunable optical filter coupled to another output of the optical splitter; (e) an optical isolator coupled to the filter; and (f) an attenuator having an input coupled to optical isolator, and an output coupled to another input of the optical combiner.
Broadly, in accordance with yet another circuit aspect of the present invention, circuitry for generating a burst support signal to augment an input optical burst signal includes: (a) a wavelength selective coupler having the input optical burst signal as one input: (b) a semiconductor optical amplifier coupled to the output of the wavelength selective coupler; (c) an optical splitter having an input coupled to the output of the optical amplifier, and an output to emit the burst support signal; (d) an optical isolator having an input coupled to the another output of the optical splitter; and (e) an attenuator having an input coupled to another output of the of the optical splitter, and an output coupled to another input of the optical combiner.